Recently, it has been more common to locate production equipment for oil and gas wells on the sea bed. Electric power is required to operate the production equipment. This requires not only electric conductors to control operations, but also heavy cables for transmission of electric power to electrical equipment, such as huge booster stations conveying recovered oil and/or gas.
Such heavy cables, normally manufactured of copper wire, are now integrated into the more traditional umbilical, which in turn is under continuous development and alters its construction and functions according to actual needs. These heavy electric cables have substantial transversal cross sections and provide the umbilical with considerable added weight due to the weight of copper. Because copper has a very poor load carrying capacity, the water depth at which the traditional umbilical can be used is very limited.
The traditional umbilicals are constructed as a composite structure which is able to transfer hydraulic liquids, chemicals, fluids, electric and optical signals and electric power between equipment on the sea bed and on the sea surface. An early version of such umbilicals is disclosed in WO 93/17176. Typically, most of the load transfer took place in a centrally located steel pipe of substantial dimension. Further examples of prior art are disclosed in GB 2 326 177 A and GB 2 326 758 A, all having the same assignee as the present invention.
It is to be understood that in the present description and patent claims, a clear distinction is made between electric cables, i.e. heavy and having a large transversal cross sectional area, and electric conductors and wires, i.e. thin and having a small cross sectional area. Each electric cable is able to transmit large quantities of electrical power, while the electric conductors or wires are used to transmit small quantities of current and control signals.
Instantly it appears that steel rope could be used as load carrying elements. This, however, does not completely solve the problem. Steel rope also adds substantial weight and, in combination with the heavy copper cables, the cables can not reach large depths before the power umbilical achieves yield stresses and are torn off due to its own weight.